BlackBerry shares tumbled Monday after Goldman Sachs Group Inc. cut its rating on the struggling smartphone maker, citing a disappointing debut for the company’s Z10 phone in the U.S.

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The shares fell more than 4.5% to US$14.23 in New York, following a decline of 7.7% on Friday. Before the two-day drop, the stock had climbed 36% this year, lifted by optimism that the company’s new lineup would fuel a turnaround.

Shares also fell about 4.5% to $14.51 on the Toronto Stock Exchange on Monday.

The Z10 went on sale last week in the U.S. — BlackBerry’s biggest single market — almost two months after its rollout in the U.K. and Canada. The country is “critical for BlackBerry’s ultimate success,” said…